376 lines
15 KiB
Text
376 lines
15 KiB
Text
COMMON SETTINGS
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==============================================================================
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Currently four types of digital filter implementations are supported. They all serve an
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individual purpose or are individual implementations of a specific filter type.
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Each filter implementation uses the same set of basic configuration tags and individual
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configuration elements. These individual elements are described in the section of the
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filter.
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The InputValue
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==============================================================================
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Each filter has several driving values, like the input value itself, sometimes a reference
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value, a gain value and others. Most of these input values can bei either a constant value
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or the value of a property. They all use the same syntax and will be referred to as InputValue
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in the remaining document.
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The complete XML syntax for a InputValue is
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<some-element>
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<property>/some/property/name</property>
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<value>0.0</value>
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<scale>1.0</value>
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<offset>0.0</offset>
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</some-element>
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The enclosing element <some-element> is the element defined in each filter, like <input>, <u_min>,
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<reference> etc. These elements will be described later.
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The value of the input is calculated based on the given value, scale and offset as
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value * scale + offset.
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With the full set of given elements, the InputValue will initialize the named property to the value
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given, reduced by the given offset and reverse scaled by the given scale.
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Example:
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<input>
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<property>/controls/flight/rudder</property>
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<value>0.0</value>
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<scale>0.5</scale>
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<offset>0.5</offset>
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</input>
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Will use the property /controls/flight/rudder as the input of the filter. The property will be initialized
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at a value of zero and since the property usually is in the range [-1..+1], the the value of <input> will
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be in the range (-1)*0.5+0.5 to (+1)*0.5+0.5 which is [0..1].
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The default values for elements not given are:
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<value/> : 0.0
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<scale/> : 1.0
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<offset/>: 0.0
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<property/> : none
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Some examples:
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<input>
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<property>/position/altitude-ft</property>
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<scale>0.3048</scale>
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</input>
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Gives the altitude in meters. No initialization of the property is performed, no offset applied.
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<reference>
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<value>0.0</value>
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</reference>
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A constant reference of zero.
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A abbreviated method of defining values exist for using a just constant or a property. The above
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example may be written as
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<reference>0.0</reference>
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Or if the reference is defined in a property
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<reference>/some/property/name</reference>
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No initialization, scaling or offsetting is performed here.
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The logic behind this is: If the text node in the element (the text between the opening and closing tag)
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can be converted to a double value, it will be interpreted as a double value. Otherwise the text will
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be interpreted as a property name.
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Examples:
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<reference>3.1415927</reference> - The constant of PI (roughly)
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<reference>/position/altitude-ft</reference> - The property /position/altitude-ft
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<reference>3kings</reference> - The constant 3. The word kings is ignored
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<reference>food4less</reference> - The property food4less
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The <property> element may also be written as <prop> for backward compatibility.
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OutputValue
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==============================================================================
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Each filter drives one to many output properties. No scaling of offsetting is implemented
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for the output value, these should be done in the filter itself.
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The output properties are defined in the <output/> element by adding <property/> elements
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within the <output/> element. For just a single output property, the <property/> element
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may be ommited. For backward compatibility, <property/> may be replaced by <prop/>.
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Nonexisting properties will be created with type double.
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Example: (Multiple output properties)
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<output>
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<property>/some/output/property</property>
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<property>/some/other/output/property</property>
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<property>/and/another/output/property</property>
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</output>
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Example: a single output property
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<output>/just/a/single/property</output>
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Other Common Settings
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==============================================================================
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<name> String The name of the filter. Used for debug purpose.
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Example:
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<name>pressure rate filter</name>
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<debug> Boolean If true, this filter puts out debug information when updated.
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Example:
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<debug>false</debug>
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<input> InputValue The input property driving the filter.
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Refer to InputValue for details.
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<reference> InputValue The reference property for filter that need one.
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Refer to InputValue for details.
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<output> Complex Each filter can drive one to many output properties.
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Refer to OutputValue for details.
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<u_min> InputValue This defines the optional minimum and maximum value the output
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<u_max> is clamped to. If neither <u_min> nor <u_max> exists, the output
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is only limited by the internal limit of double precision float computation.
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If either <u_min> or <u_max> is given, clamping is activated. A missing
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min or max value defaults to 0 (zero).
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Note: <u_min> and <u_max> may also occour within a <config> element.
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Example: Limit the pilot's body temperature to a constant minimum of 36 and a maximum defined in
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/pilots/max-body-temperature-degc, initialized to 40.0
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<u_max>
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<prop>/pilots/max-body-temperature-degc</prop>
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<value>40.0</
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</u_max>
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<u_min>
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<value>36.0</value>
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</u_min
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Implicit definition of the minimum value of 0 (zero) and defining a maximum of 100.0
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<config>
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<u_max>100.0</u_max>
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</config>
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<enable> Complex Define a condition to enable or disable the filter. For disabled
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filters, no output computations are performed. Only enabled
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filters fill the output properties. The default for undefined
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conditions is enabled.
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Several way exist to define a condition. The most simple case
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is checking a boolean property. For this, just a <prop> element
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naming this boolean property is needed. The boolean value of the
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named property defines the enabled state of the filter.
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To compare the value of a property with a constant, a <prop> and
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a <value> element define the property name and the value to be
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compared. The filter is enabled, if the value of the property
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equals the given value. A case sensitive string compare is
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performed here.
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To define more complex conditions, a <condition> element may be
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used to define any condition described in README.conditions.
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If a <condition> element is present and if it contains a valid
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condition, this conditions has precedence over a given <prop>/
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<value> condition.
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The child element <honor-passive>, a boolean flag, may be present
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within the <enable> element. If this element is true, the property
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/autopilot/locks/passive-mode is checked and if it is true, the
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filter output is computed, but the output properties are not set.
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The default for honor-passive is false
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Example: Check a boolean property, only compute this filter if gear-down is true and
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/autopilot/locks/passive-mode is false
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<enable>
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<prop>/gear/gear-down</prop>
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<honor-passive>true</honor-passive>
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</enable>
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Check a property for equality, only compute this filter if the autopilot is locked in heading mode.
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<enable>
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<prop>/autopilot/locks/heading</prop>
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<value>dg-heading-hold</value>
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</enable>
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Use a complex condition, only compute this filter if the autopilot is serviceable and the lock
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is either dg-heading-hold or nav1-heading-hold
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<enable>
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<condition>
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<property>/autopilo/serviceable</property>
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<or>
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<equals>
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<property>/autopilot/locks/heading</property>
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<value>dg-heading-hold</value>
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</equals>
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<equals>
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<property>/autopilot/locks/heading</property>
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<value>nav1-heading-hold</value>
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</equals>
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</or>
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</condition>
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</enable>
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INDIVIDUAL FILTER CONFIGURATION
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==============================================================================
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Digital Filter
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Six different types of digital filter can be configured inside the autopilot
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configuration file. There are four low-pass filter types and two gain filter
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types.
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The low-pass filter types are:
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* Exponential
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* Double exponential
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* Moving average
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* Noise spike filter
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The gain filter types are:
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* gain
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* reciprocal
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To add a digital filter, place a <filter> element under the root element. Next to
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the global configuration elements described above, the following elements configure
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the digital filter:
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<filter-time> InputValue This tag is only applicable for the exponential and
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double-exponential filter types. It controls the bandwidth
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of the filter. The bandwidth in Hz of the filter is:
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1/filter-time. So a low-pass filter with a bandwidth of
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10Hz would have a filter time of 1/10 = 0.1
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<samples> InputValue This tag only makes sense for the moving-average filter.
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It says how many past samples to average.
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<max-rate-of-change>
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InputValue This tag is applicable for the noise-spike filter.
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It says how much the value is allowed to change per second.
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<gain> InputValue This is only applicable to the gain and reciprocal filter types.
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The output for gain filter is computed as input*gain while
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the reciprocal filter computes output as gain/input for input
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values != 0 (zero). Gain may be a constant, a property name
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defined by a <prop> element within the <gain> element or a
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property name initialized to a value by using a <prop> and
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<value> element.
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Example: a pressure-rate-filter implemented as a double exponential low pass filter
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with a bandwith of 10Hz
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<filter>
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<name>pressure-rate-filter</name>
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<debug>false</debug>
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<type>double-exponential</type>
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<enable>
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<prop>/autopilot/locks/pressure-rate-filter</prop>
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<value>true</value>
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</enable>
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<input>/autopilot/internal/pressure-rate</input>
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<output>/autopilot/internal/filtered-pressure-rate</output>
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<filter-time>0.1</filter-time>
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</filter>
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This will filter the pressure-rate property. The output will be to a new
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property called filtered-pressure-rate. You can select any numerical property
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from the property tree. The input property will not be affected by the filter,
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it will stay the same as it would if no filter was configured.
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Example 2:
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<filter>
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<name>airspeed elevator-trim gain reciprocal filter</name>
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<debug>false</debug>
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<enable>
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<prop>/autopilot/locks/airspeed-elevator-trim-gain</prop>
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<value>true</value>
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</enable>
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<type>reciprocal</type>
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<gain>
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<prop>/autopilot/settings/elevator-trim-airspeed-reciprocal-gain</prop>
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<value>7</value>
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</gain>
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<input>/velocities/airspeed-kt</input>
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<output>/autopilot/internal/elevator-trim-gain</output>
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<u_min>0.005</u_min>
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<u_max>0.02</u_max>
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</filter>
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This will use the /velocities/airspeed-kt property to produce a gain factor
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that reduces as airspeed increases. At airspeeds up to 350kt the gain will
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be clamped to 0.02, at 700kt the gain will be 0.01 and at 1400kt the gain will
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be 0.005. The gain will be clamped to 0.005 for airspeeds > 1400kt.
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The output from this filter could then be used to control the gain in a PID
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controller:
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<pid-controller>
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<name>Pitch hold</name>
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<debug>false</debug>
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<enable>
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<prop>/autopilot/locks/pitch</prop>
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<value>true</value>
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</enable>
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<input>
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<prop>/orientation/pitch-deg</prop>
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</input>
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<reference>
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<prop>/autopilot/settings/target-pitch-deg</prop>
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</reference>
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<output>
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<prop>/autopilot/internal/target-elevator-trim-norm</prop>
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</output>
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<config>
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<Ts>0.05</Ts>
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<Kp>
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<prop>/autopilot/internal/elevator-trim-gain</prop>
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<value>0.02</value>
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</Kp>
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<beta>1.0</beta>
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<alpha>0.1</alpha>
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<gamma>0.0</gamma>
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<Ti>2.0</Ti>
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<Td>0.2</Td>
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<u_min>-1.0</u_min>
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<u_max>1.0</u_max>
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</config>
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</pid-controller>
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IMPORTANT NOTE: The <Kp> tag in PID controllers has been revised to operate in
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the same way as the <gain> elements in filters. However, the original format
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of <Kp> will continue to function as before i.e. <Kp>0.02</Kp> will specify a
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fixed and unalterable gain factor, but a warning message will be output.
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The gain type filter is similar to the reciprocal filter except that the gain
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is applied as a simple factor to the input.
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-------------------------------------------------------------------------------
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Parameters
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<name> The name of the filter. Give it a sensible name!
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<debug> If this tag is set to true debugging info will be printed on the
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console.
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<enable> Encloses the <prop> and <value> tags which are used to enable or
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disable the filter. Instead of the <prop> and <value> tags, a <condition>
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tag may be used to define a condition. Check README.conditions for more
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details about conditions. Defaults to enabled if unspecified.
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<type> The type of filter. This can be exponential, double-exponential,
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moving-average, noise-spike, gain or reciprocal.
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<input> The input property to be filtered. This should of course be a
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numerical property, filtering a text string or a boolean value does not make
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sense.
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<output> The filtered value. You can make up any new property.
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<u_min> The minimum output value from the filter. Defaults to -infinity.
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<u_max> The maximum output value from the filter. Defaults to +infinity.
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These are the tags that are applicable to all filter types. The following tags
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are filter specific.
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<filter-time> This tag is only applicable for the exponential and
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double-exponential filter types. It controls the bandwidth of the filter. The
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bandwidth in Hz of the filter is: 1/filter-time. So a low-pass filter with a
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bandwidth of 10Hz would have a filter time of 1/10 = 0.1
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<samples> This tag only makes sense for the moving-average filter. It says how
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many past samples to average.
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<max-rate-of-change> This tag is applicable for the noise-spike filter. Is
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says how much the value is allowed to change per second.
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<gain> This, and it's enclosed <prop> and <value> tags, are only applicable to
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the gain and reciprocal filter types. The <prop> tag specifies a property node
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to hold the gain value and the <value> tag specifies an initial default value.
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The gain defaults to 1.0 if unspecified.
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The output from the gain filter type is: input * gain.
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The output from the reciprocal filter type is: gain / input.
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The gain can be changed during run-time by updating the value in the property
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node.
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